The invention relates to a process that eliminates constraints on proven NOx and SO3 reduction technology, by providing a specialized treatment with efficiently controlled reagent introduction for maintaining economy while addressing serious emissions control problems.
Combustion of carbonaceous fuels almost invariably results in pollution. Regulation of the quality of the emissions from combustion sources is essential for maintaining the quality of the air we require for survival. The technology for treating emissions and for reducing the generation of harmful gases has been greatly advanced towards meeting the often opposed objectives of clean air and reasonable costs. Unfortunately, some technological solutions have been shown to be competitive with each other. In these cases, implementation of them at the same time is often too expensive or technically complicated, with the result that old plants or ones with insufficient space availability are shut down or derated. Economic operation of power plants and incinerators is in the public interest, and new technologies are essential to this effort.
The selection of fuels like natural gas can reduce some pollution problems, but it cannot eliminate them. Nitrogen oxides (NOx) are invariably formed with combustion and are often treated by selective non catalytic reactions (SNCR) or selective catalytic reactions (SCR). Burning other fuels, like No. 6 oil, will create NOx and can cause other problems for boiler operators—including high temperature slagging/fouling and related eutectic corrosion, cold end corrosion/fouling and opacity issues related to carbon particulate and acid mist. In the combustion zone, sulfur in the oil (e.g., 1-5%) auto-catalyzes to sulfur trioxide (SO3), which can condense as sulfuric acid on the back end surfaces (where the temperature has typically been reduced to less than about 150° C.) and promote corrosion and acid plume. In addition, SO3 can be result from oxidation by SCR catalysts.
For SO3 control, injection of alkali material such a magnesium hydroxide is useful; but it typically results in accumulation of solids along the wall and floor due to inadequacies in material properties, equipment design, and injection process. Solids accumulation may lead to an outage of a combustor or a process. Solids accumulation also leads to inefficient use of the reagent. Even with an SO3 control reagent in the fuel or injected into the combustion gases, SO3 remains; and the effluent reaching the cold end can cause problems due its acid pH and the presence of too much SO3. The low pH can adversely affect fly ash disposal and cold end corrosion.
SO3 vapor readily converts to gaseous sulfuric acid when combined with water vapor in the flue gas. As gas and surface temperatures cool through the system the SO3, vapors form a fine aerosol mist of sulfuric acid. The acid aerosol contains sub-micron particles of acid, which can evade separation or capture in gas cleaning devices and exit the stack. Even relatively low SO3 concentrations exiting the stack cause significant light scattering and can easily create a visible plume and high opacity reading. As a general rule, every 1 part per million by volume of SO3 will contribute from 1 to 3% opacity. Thus, exhaust gas concentrations of only 10 to 20 ppm SO3 can cause opacity and acid plume problems. In addition, deposition or formation of acid on any metal surfaces below the acid dew point causes corrosion within the unit, such as at the air heater, duct work and stack liners.
The presence of an SCR unit can further exacerbate the SO3 problem by oxidizing SO2 to SO3. It is not uncommon for the SO3 levels to double (or more) across the SCR catalyst. In a typical SCR NOx reduction system, the effluent containing NOx is passed over a suitable catalyst which reduces the NOx to nitrogen (N2) and water (H2O) by a reagent comprising ammonia (NH3), urea [(NH2)CO(NH2)], or the like. The catalyst effective to reduce the NOx in the presence of these reagents, also strongly promotes the oxidation of SO2 to SO3. In some cases, SO2 can also be oxidized to SO3 by other equipment. There is a clear need to reduce NOx, but the SO3 burden created by an SCR or other oxidizing unit must also be controlled.
SCR units are large and costly. To be effective, they must operate at relatively low temperatures and often fill all available space between the combustor and an air heater which uses residual heating capacity of the effluent to heat incoming combustion air. Because of the typical low temperature operation and the presence of significant SO3 concentrations following an SCR unit, it is sometimes necessary to heat the effluent to avoid corrosion, plume, opacity and related problems. Heating in this manner is a further source of inefficiency, and it would be beneficial if there were a way to avoid it.
Historically, SO3 has been reduced by introducing an SO3 treatment agent like magnesium hydroxide at appropriate positions in the duct work. Not all alkaline treatment agents will be useful because SO3 also reacts with water vapor and ammonia used for the SCR reaction to form ammonium sulfate and ammonium bisulfate. Both of these ammonia salts can cause fouling and corrosion problems in the system. Ammonium bisulfate has a melting point under 300° F. and ammonium sulfate at just over 450° F., making both molten or tacky at typical SCR and air heater operating temperatures and making it possible for them to coat, foul and corrode the air heater. Lime cannot be practically used to eliminate the SO3 because it reacts to form gypsum, which can also create fouling problems. Gypsum forms a hard, non-friable deposit with very low solubility that is difficult to remove. Magnesium hydroxide can be better from this standpoint, but has not been introduced with effectiveness downstream of the catalyst, due to particle size and distribution problems.
There is a need for an improved process that could improve the compatibility of SCR treatments for sulfur containing fuels and more effectively deal with back end SO3 corrosion.